Tone languages such as Thai use pitch differences to distinguish lexical meaning. Previous behavioral studies have reported that naïve listeners can discriminate among lexical tones, but that native language background affects performance. The present study uses ERPs to determine whether native speakers of a tone language (Mandarin Chinese) and of a non-tone language (English) differ in their pre-attentive discrimination among Thai lexical tones, and whether training has a different effect in these two language groups. EEGs were obtained from 10 native Mandarin Chinese speakers, 10 English and 10 Thai speakers in an oddball paradigm: The Thai syllable [k(h)a:] pronounced with a high rising or low falling tone, was presented as an infrequent deviant amidst a standard mid level tone [k(h)a:] syllable, while participants watched a silent movie. Next, the Chinese and English participants completed a 2-day perceptual identification training on the mid level and low falling tones, and returned for a post training EEG. The low falling tone deviant elicited a Mismatch Negativity (MMN) in all participant groups before and after training; the high rising deviant elicited no, or a smaller, MMN, which became larger after training only in the English group. The high rising deviant also elicited a later negativity (350-650 ms) versus the mid level standard, which decreased after training in the Chinese group. These results suggest that non-Thai speakers can pre-attentively discriminate among Thai tones, but are sensitive to different physical properties of the tones, depending on their native language. English speakers are more sensitive to early pitch differences, whereas native speakers of Mandarin Chinese are more sensitive to the (later) pitch contour.